This invention relates generally to micromechanical elements, such as movable joints, gears, and sliders and more particularly concerns fabrication methods for making high precision micromechanical elements.
Micromechanical elements with joints that slide or rotate have many uses in miniature pin joints, gears, cranks, slides, and other mechanisms. These elements can be made in a variety of ways. In U.S. Pat. No. 4,740,410 by Muller et al., micromechanical sliding or rotating elements are made by following the process steps of:
1) depositing a sacrificial layer of glass on a substrate,
2) depositing and forming a structural layer of polysilicon for the sliders or gears,
3) depositing a second sacrificial layer of glass,
4) depositing and forming a second structural layer of polysilicon for the rails or pins,
5) removing the sacrificial layers to free the gears and sliders from both the substrate and the pins and rails.
This process results in fixed axle pin joints or fixed rail sliders. Once loosened, the fixed gears and sliders rest on the substrate. In operation, undesirable amounts of friction are generated between the gears and sliders, and the substrate.
A process for making self-constraining joints is also disclosed in Muller et. al. Self-constraining joints may slide and rotate at the same time. These joints are constructed using a small variation of the basic process discussed above. The self-constraining joints are differentiated from the fixed joints by constructing a flange on the pins and rails underneath the gears and sliders to keep them in place. The pins and rails can either be fixed to the substrate or left free to slide across the substrate. The pins and rails are constructed using a portion of the normally first sacrificial glass layer to form the pin or rail and the first structural polysilicon layer to form the flange. The flange is formed with an etch undercut process. Etch undercutting processes are in general not easily controllable. The glass/polysilicon joint is also the weakest part of the structure and tends to break under stress.
Further information about this process can also be found in a paper by Muller et al:
"Integrated Movable Micromechanical Structures for Sensors and Actuators", IEEE Transactions on Electron Devices Vol. 35, No. 6, June 1988.